The rapidly expanding elderly population and associated health care costs highlight the need to develop interventions that promote healthy aging. Many older adults are advised to perform moderate intensity aerobic exercise for its benefits on cardiovascular health, glucose regulation, and weight management. Despite the prevalence of aerobic exercise, very little is known about the influence of this form of exercise on muscle function. Therefore the aim of this study is to comprehensively assess the effects of aerobic exercise training on skeletal muscle size and contractile function at both the whole muscle and single muscle fiber levels in older men and women. To achieve this aim, we will recruit 26 individuals, 13 men and 13 women, age 65-80 yr, who will perform 12-weeks of progressive aerobic exercise training on a cycle ergometer. Before and after a 12-week free-living control period and again after the training intervention, subjects will undergo magnetic resonance imaging (MRI) for the determination of muscle size, perform muscle function testing to determine peak strength and power production of the knee extensors, and undergo a muscle biopsy procedure for the analysis of single muscle fiber contractile physiology and the determination of myosin heavy chain morphology. Based on our preliminary data, we hypothesize that aerobic exercise training will induce muscle hypertrophy and improve whole muscle function. We further hypothesize that the improvements in whole muscle size and function will be explained by a remodeling of the contractile mechanics (i.e., peak force and contraction velocity) of slow and fast-twitch muscle fibers, ultimately resulting in a greater peak power production. Dr. Harber, a new investigator, and the assembled investigative team collectively have extensive experience implementing the proposed experimental procedures and analytical techniques in older adults. This investigation will provide novel insights into the regulation of muscle plasticity by physical activity in the elderly population. Furthermore, these data will contribute to the improvement of the health and well-being of older Americans by enhancing our understanding of therapeutic interventions that preserve and/or increase muscle mass and functional mobility. PUBLIC HEALTH RELEVANCE: This research will comprehensively assess the effects of aerobic exercise training on muscle size and function in older men and women. Outcomes from these studies may improve the current exercise-based therapeutic approach to the treatment of sarcopenia by including aerobic exercise as an effective modality for enhancing muscle mass, function, functional mobility, and overall quality of life in older adults.